rpmsg-kdrv: fix 'variable 'ret' is used uninitialized whenever 'for' loop exits'

Fix build warning
rpmsg_kdrv.c:282:14: warning: variable 'ret' is used uninitialized whenever 'for' loop exits because its condition is false [-Wsometimes-uninitialized]
        for (i = 0; i < priv->num_raw_devices; i++)

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>